Not sure if anyone else has done this or not; thought I'd start a new thread.
Bought an Apex the other day after reading all the posts about it, and hearing that my pal was getting one. Can't be outdone, ya know! I wanted one as a backup for my Sten that I could mount on the helmet. I never have liked the idea of those elastic straps holding on a light, so I set about making it permanent.
I removed the two screws on the top of the battery compartment, exposing the solder points. Desoldered same and fished the cable through several holes in my helmet. I also covered the wire with cable wrap. This seems to help protect the exposed parts outside the helmet.
Five drilled holes and some nuts and bolts later, I had it all rigged up. I can still aim the light to correspond with the Sten.
Just for perusal!